A Study to Evaluate Mosunetuzumab in Participants With Systemic Lupus Erythematosus With or Without Active Lupus Nephritis

This study will assess how mosunetuzumab works in people who have systemic lupus erythematosus (SLE) who may or may not also have active lupus nephritis (LN).

Inclusion Criteria: * Diagnosis of SLE for ≥ 6 months as assessed using the 2019 European League Against Rheumatism/American College of Rheumatology (EULAR/ACR) Classification Criteria at screening Exclusion Criteria: * Pregnant or breastfeeding, or intention of becoming pregnant during the study or within the time frame in which contraception is required * Treatment with investigational therapy within 30 days or 5 drug-elimination half-lives (whichever is longer) prior to initiation of study treatment and during the study * Major surgery requiring hospitalization during the 4 weeks prior to screening or during screening, or any planned surgery or procedure requiring hospitalization during the 12 weeks following study drug administration * Alcohol or substance abuse within the 12 months prior to screening * Active infection of any kind, excluding fungal infection of the nail beds * Any major episode of infection as defined by the protocol * History of serious recurrent or chronic infection * History of progressive multifocal leukoencephalopathy (PML) * Tuberculosis (TB) infection * History of cancer, including solid tumors, hematological malignancies, and carcinoma in situ, within the past 5 years * Active overlap syndrome with mixed connective tissue disease or systemic sclerosis within the 12 months prior to screening or during screening * Catastrophic or severe antiphospholipid syndrome within the 12 months prior to screening or during screening. Antiphospholipid syndrome adequately controlled by anticoagulant therapy for at least 2 months prior to screening is acceptable * High risk for clinically significant bleeding or any condition requiring plasmapheresis, IV immunoglobulin, or acute blood product transfusions * Active severe or unstable lupus-associated neuropsychiatric disease or where, in the opinion of the investigator, it is likely to require treatment with protocol-disallowed therapies. Examples of neuropsychiatric SLE manifestations include, but are not limited to the following: meningitis, retinitis, cerebral vasculitis, myelopathy, demyelination syndromes, acute confusional state, psychosis, acute stroke or stroke syndrome, cranial neuropathy, status epilepticus or seizures, cerebellar ataxia, and mononeuritis multiplex * History of any non-SLE disease treated with oral, intravenous, or intramuscular corticosteroids for more than 14 days in total during the one year prior to Day 1 * History of treatment with any T cell-engaging bispecific antibodies or CAR-T therapy within the past 2 years * Receipt of any live or attenuated vaccine in the 28 days prior to or during screening